generally, the higher fin-per-inch (more fins - higher density) would require a
taller RPM fan speed. the looser the fin array the easier the air can pass-through
and use lower fan-speed according to desired temperatures needed.

i will always error to the RX-series, unless space dictates otherwise.
but push/pull on the AX/EX-series will make the build quieter because the
fan only has to work half as hard over the RX radiator. so two low speed fans
over one medium fan speed usually triumphs in low noise output.